Galactomannans are a class of polysaccharides composed of galactose and mannose. In general, the structure of the polymers consist of a linear chain of 1,4-linked beta D-mannopyranosyl units with single-membered alpha-D-galactopyranosyl units linked through the 1,6 positions. Galactomannans vary in the extent and uniformity of substitution with respect to their galactose moieties For example, the two most common galactomannans are guar and locust bean gums. Guar has a galactoae:mannose ratio approximating 1:2 locust bean gum has a ratio of about 1:4. Both are soluble in water, but the greater number of galactose units on guar imparts better cold water solubility; locust bean gum requires heating to dissolve thoroughly.
Water-soluble other derivatives of galactomannans are also well-known and commercially available--common examples being carboxymetyl-, hydroxypropyl-, and hydroxyethyl-. Such derivatives contain relatively low degrees of substitution (DS) of the modifying reagent, generally less than 1.0 DS (out of a maximum DS of 3), and are not soluble in common or organic solvents. These modifying groups are added at low levels to alter aqueous solution rheology or impart ionic character to the galactomannan.
High DS etherified and/or esterified galactomannans are described in Australian Patent AU-636800-B (and its counterpart German Patent Application P 40 33 041.9). These derivatives are substituted to DS values greater than about 2.4 with C.sub.1 -C.sub.6 alkoxy, phenyl C.sub.1 -C.sub.4 alkoxy or phenoxy groups, and ester groups of C.sub.3 -C.sub.16 alkanoyloxy groups. These derivatives are soluble in organic solvents, and are claimed to be useful in pharmaceutical preparations. However, the process used to make the high DS products is extremely laborious, complicated, and uneconomical. Examples given for preparing methylated and ethylated derivatives describe a process wherein a protein-free galactomannan is first dissolved in water and reprecipitated in methanol. After filtration, the wet-cake is added to dimethylsulfoxide and the residual methanol is aspirated off. Solid sodium hydroxide in then added followed by the modifying alkylating reagent, such as methyl iodide or ethyl bromide. The mixture is stirred at 20.degree.-25.degree. C. for 48 hours. It is stated that reaction temperatures higher than about 35.degree. C. are undesirable because of the formation of yellow colored byproducts. Undissolved material is then separated and the supernatant treated in a rotary evaporator to remove dimethylsulfoxide. The residue is then treated with ether, filtered to remove undissolved material, then concentrated on a rotary evaporator until the polymer precipitates. The polymer is filtered, dissolved in methanol, reprecipitated in hot water, and finally dried.
U.S. Pat. No. 4,169,945 describes another process for making alkyl ethers of galactomannans specifically requiring the presence of a quaternary ammonium halide phase-transfer reagent in the reaction. Though this patent claims that the process is suitable for preparing galactomannans having degrees of substitution between 0.01 to 3.0, the examples are only enabling for low DS water soluble products, i.e., DS less than about 1.0. The sentence in lines 24-28, column 4, attests to the fact that their process is specific to producing water soluble derivatives: "If more than the optimum quantity of water is present in this type of reaction medium, then the polygalactomannan gum may swell or enter solution, thereby complicating product recovery and purification". The process disclosed in this patent may be conducted in water or in a two phase reaction system comprising water and a water miscible solvent.